Miklós Görgényi

Principal component analysis of Kovats indices for carbonyl compounds in capillary gas chromatography

Abstract Principal component analysis was performed on a data matrix consisting of Kovats indices of 35 aliphatic ketones and aldehydes. The calculations were carried out on the correlation matrices of Kovats indices. The Kovats indices were determined on capillary columns with four different stationary phases, namely bonded methyl- (HP-1), methylphenyl- (HP-50), and trifluoropropylmethylsiloxane (DB-210), as well as polyethylene glycol (HP-Innowax) at four different temperatures. It was found that one principal component accounts for more than 94% of the total variance in the data, indicating that the temperature does not change the dominant pattern in the data. The physical meaning attributable to the principal components, thus the most influential ones are as follows: the first principal component accounts for the boiling point (and/or the molecular mass) of carbonyl compounds whereas the second is responsible for the temperature dependence. The plots of component loadings showed a characteristic pattern (counterclockwise increasing temperature) whereas that of component scores showed a triangular structure and some groupings of oxo compounds. Abstract retention data (free from influence of temperature and column polarity) are non-linear functions of boiling points of solutes. Similarity among the solutes from the point of view of retention is represented by characteristic plots.

Correlation between retention indices and quantum-chemical descriptors of ketones and aldehydes on stationary phases of different polarity

Abstract Kovats retention indices determined on four different capillary columns (OV-1, HP-50, DB-210 and HP-Innowax) were correlated with molecular structural parameters calculated by a semiempirical quantum-chemical method PM3. Multivariate techniques: principal component analysis and cluster analysis were applied to extract the data structure. Multiple linear regression was made in forward stepwise manner to select suitable variables in the model. Basic correlations were found between the retention indices on different columns and the molecular surface, energies of highest occupied and lowest unoccupied molecular orbitals { E (HOMO) and E (LUMO)}, polarizability, and dipole moments. These correlations provide insights into the mechanism of chromatographic retention on a molecular level. They support the view: the more polar is the stationary phase, the greater is the effect of the polarity (polarizability and dipole moment) of solute molecules on the retention phenomena.

Temperature dependence of Kováts indices in gas chromatography revisited

Temperature dependence of the Kováts retention index (I) was measured for some aliphatic ketones and aldehydes on a poly(dimethyl siloxane) (HP-1) stationary phase. An interesting minimum (non-linearity) was observed for the I versus isothermal column temperature (T) relationships. A novel empirical model is proposed: I=A+B/T+C ln T, where A, B and Care equation constants and B/C = T(min). A detailed statistical analysis clearly shows superiority of the extended model (i.e., of this containing the logarithm of the temperature (ln T) term) over the earlier established Antoine-type reciprocal equation. The minimum temperature (and the energy like quantity=RT(min), where R is the gas constant) changes in a systematic manner. The factors effecting the (RT(min)) term are as follows: (i) this term decreases with the increase of the molecular mass of the respective oxo compounds; (ii) ketones have higher absolute values of (RT(min) than aldehydes; (iii) branching of the carbon chain lowers the mentioned (RT(min)). This enthalpy term is unambiguously bound to the polarity of solutes.

Temperature dependence of Henry's law constant in an extended temperature range

The Henrys law constants H for chloroform, 1,1-dichloroethane, 1,2-dichloropropane, trichloroethene, chlorobenzene, benzene and toluene were determined by the EPICS-SPME technique (equilibrium partitioning in closed systems--solid phase microextraction) in the temperature range 275-343 K. The curvature observed in the ln H vs. 1/T plot was due to the temperature dependence of the change in enthalpy delta H0 during the transfer of 1 mol solute from the aqueous solution to the gas phase. The nonlinearity of the plot was explained by means of a thermodynamic model which involves the temperature dependence of delta H0 of the compounds and the thermal expansion of water in the three-parameter equation ln (H rho TT) = A2/T + BTB + C2, where rho T is the density of water at temperature T, TB = ln(T/298) + (298-T)/T, A2 = -delta H298(0)/R, delta H298(0) is the delta H0 value at 298 K, B = delta Cp0/R, and C2 is a constant. delta Cp0 is the molar heat capacity change in volatilization from the aqueous solution. A statistical comparison of the two models demonstrates the superiority of the three-parameter equation over the two-parameter one ln H vs. 1/T). The new, three-parameter equation allows a more accurate description of the temperature dependence of H, and of the solubility of volatile organic compounds in water at higher temperatures.

Partial least squares modeling of retention data of oxo compounds in gas chromatography

SummaryPartial least squares modeling of latent structures were carried out on a data matrix consisting of Kováts retention indices of 35 aliphatic ketones and aldehydes and their physical characteristics. The retention indices were determined on capillary columns with 4 different stationary phases, namely bonded methyl-{HP-1}, methyl-phenyl- {HP-50} and trifluoropropyl-methyl siloxane {DB-210}, as well as polyethylene glycol {HP-Innovax} at four different temperatures. It was found that ketones and aldehydes cannot be classified on the basis of retention data solely, whereas the physical characteristics (boiling point, molar volume, molecular mass, molar refraction, octanol-water partition coefficient) contain the necessary information for differentiation of the two classes of compounds. The retention index of but-2-enal does not fit into the general trend of aliphatic aldyhydes and ketones. A predictive PLS model was built to estimate retention data of oxo compounds at different temperatures and various stationary phases of different polarity. Cross-validation suggests a good reliability of the results. Characteristic plots (PLS weights, scores) show the similar retention behavior of oxo compounds (Figures 3 and 4).

Correlation of retention indices with van der Waals' volumes and surface areas: Alkanes and azo compounds

SummaryVan der Waals’ volumes (VW) and surface areas (SW) of alkanes, (E)-azoalkanes and structurally similar alkenes (R1-X=X-R2, X=N, CH) were calculated by a semiempirical quantum-chemical method (AM1). The calculated data are in reasonable agreement with the experimental values of Bondi and good correlations were found between the calculated data and Kovats’ retention indices (IR). While theVWs of alkanes with the same carbon number are very close to one another, theSWs follow the scatter of theIR values for branched alkanes. The difference in theIR of (E)-azo compounds and the structurally similar alkenes can be explained by the difference inVWs.

Minimum in the temperature dependence of the Kováts retention indices of nitroalkanes and alkanenitriles on an apolar phase

The Kováts retention indices (I) of 1-nitroalkanes and alkanenitriles were determined on polydimethylsiloxane and Innowax (polyethylene glycol) columns in a wide temperature range. The temperature dependence of the retention indices exhibits a definite minimum for the early members of the homologous series. The position of the minimum shifts to lower temperatures with increasing carbon atom number of the solute. The thermodynamic explanation of an extreme in the I vs. T function is the higher solvation heat capacities of nitroalkanes and alkanenitriles relative to those of the reference n-alkanes, owing to the deviation from the ideal state in the solution. A novel equation was derived which describes the minimum in the I vs. T function, too.

Estimation and prediction of the retention indices of selected trans-diazenes

SummaryThe gas chromatographic retention indices of some typicaltrans-diazenes were determined on a methyl silicone capillary column. Thetrans-diazenes are less strongly retained than the analogous olefins. Correlations of the retention indices with physico-chemical parameters, and of the boiling points with topological indices were also determined.

Prediction of gas chromatographic retention indices of 2,4-dinitrophenylhydrazones

Abstract Kovats retention indices ( I ) of C 1 –C 8 aldehydes and ketones and their 2,4-dinitrophenylhydrazones (DNPHs) were determined. The temperature gradient of the retention indices of keto hydrazones is higher than that of aldehyde hydrazones, hence the separation of certain co-eluting hydrazones is possible by changing the elution temperature. The retention index difference between the E - and Z -isomers increases with increasing asymmetry in the groupss attached to the hydrazo carbon atom. The slope a in the equation I DNPH = aI oxo + b is near unity for not heavily branched aldehyde DNPHs, whereas for keto E -isomers it is leess than unity and changes with the structure.

Matrix effect in the cryotrapping of gas samples in the ppbv range

A gas mixture containing ppbv concentrations of volatile chlorinated and aromatic hydrocarbons was prepared in helium, nitrogen, or argon in a plastic (Nalophan) bag, in which it was found the mixture could be safely stored for several days. Samples of the mixture were subsequently analyzed by gas chromatography after direct cryotrapping in an empty metal U-tube. The cooling medium used was liquid nitrogen, liquid argon, or a dry ice-acetone slurry (197 K). The efficiency of cryotrapping in liquid nitrogen was over 90% when the aromatic hydrocarbon mixture was prepared in the helium matrix, but between 50 and 70% when it was prepared in nitrogen or argon, the recovery from the argon matrix being somewhat higher. The poor recovery in nitrogen at 77 K and argon at 87 K was explained by the reduced rate of diffusion to the wall owing to mist or aerosol formation. At 197 K condensation was negligible if the partial pressures were lower than the saturated vapor pressures.